One of the challenges of the multi-mode and multi-standard wireless communication systems of today is the integration of large numbers of filters and transistor circuits. Therefore, a multi-band filter bank module fabricated in a single chip is the only solution to the aforementioned problem.
The conventional microelectromechanical system (MEMS)-based filter usually comprises two types of structures. One is a film bulk acoustic-wave resonator (FBAR) structure and another is a thickness field excitation (TFE) resonator structure. For the conventional FBAR MEMS-based filter, the direction of the electrode field (Efield) that is generated is parallel to a strain direction of the piezoelectric material, so that the conventional FBAR MEMS-based filter utilizes the piezoelectric coefficient D33. The operation frequency of the conventional FBAR MEMS-based filter, however, is defined by the film thickness of the piezoelectric material. The piezoelectric materials of the conventional FBAR MEMS-based filters cannot be deposited as films with various thicknesses on a surface of a single substrate due to limitations of the conventional thin-film process. Therefore, the conventional FBAR MEMS-based filter cannot achieve the goal of having a multi-band filter bank module in a single chip Additionally, the direction of the electrode field (Efield) generated by the conventional TFE MEMS-based filter is parallel to the strain direction of the piezoelectric material of the conventional TFE MEMS-based filter, so that the conventional TFE MEMS-based filter utilizes the piezoelectric coefficient D31. The electro-mechanical conversion efficiency of the conventional TFE MEMS-based filter is reduced. Therefore, the conventional TFE MEMS-based filter cannot satisfy the communication system specification of today due to its disadvantages, for example, high input impedance and narrow bandwidth.
Thus, a novel MEMS-based resonator device having a high electro-mechanical coupling coefficient (Keff2) and low impedance is desirable to achieve a goal of a multi-band filter module in a single chip.